Mineral Processing - Crushing - Plant design, construction, operation and optimisation

Title

Pag

e

Crushing / Screening and Conveying

Basdew RooplalMetallurgical Consultant 1

PLANT DESIGN CONSTRUCTION AND OPERATIONPLANT OPTIMISATION AND ENERGY EFFICIENCY CONSIDERATIONS

Cont

ents

ContentsPlant Design Construction and

Operation

Bench scale and pilot scale design for comminution circuits

Factors influencing the selection of comminution circuits

Types and characterisation of crusher equipment and circuit flowsheet

Selection and sizing of primary crushero Computer aided design of Jaw

Crusher

Selection and sizing of secondary and tertiary crusherso Optimising the Eccentric

speed of cone crusher

Selection and sizing of High pressure roll crushers

Advancement in Screening Technology. 2

Cont

ents

ContentsPlant optimisation and energy

efficiency considerations

• Characterisation – Understanding the ore body and the Metallurgy

• Ore dressing studies – what is involved.

• Blasting for improved mining and comminution productivity

• Production planning for the combined mine and comminution operation

• Optimising lumps to fines ratio in Iron Ore processing• Reducing fines

generation in Coal Mining• Profit based

comminution controls• Increasing the energy

efficiency of Processing 3

BENCH SCALE AND PILOT SCALE DESIGN FOR COMMINUTION CIRCUITS

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Bench Scale testwork

Introduction

• The resistance of ore samples to breakage (or hardness) is measured through grindability tests.

• Several grindability tests have been developed over the years for different applications and each test has its own strengths and weaknesses

• Grindability tests are a compromise between test costs and its deliverables.

• The highest degree of deliverables and certainty is achieved in a pilot plant, which is also the most reliable test procedure to determine the resistance of ore samples to grinding or hardness and is also the most expensive. 5

Summary of Grindability tests

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Grindability tests

Bond Ball mill Grindability

• The AG/SAG mill or HPGR circuit products, which have non-standard particle size distribution.• One of the keys of the

Bond work index success over time has been its reliability and reproducibility.

• The figure below shows that the Ball Mill work index is normally distributed with AVG 14.6 and Median 14.8

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Grindability Tests

Bond Rod mill work Index

• The rod mill work Index is also normally distributed with and average and median of 14.8kWh/t• It is common to observe

difference between the ball and rod mill caused by variation in ore hardness

• The test has been mainly used for the design of rod mill or primary ball mills.

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Grindability tests

Bond low energy impact test

• Consists of an apparatus with two pendulum hammers mounted on two bicycle wheels, so as to strike equal blows simultaneously on opposite sides of each rock specimen.

• The height of the pendulum is raised until the energy is sufficient to break the rock specimen

• The test is generally performed on 20 rocks

• One of the strengths of the test is to measure the natural dispersion in the sample.

• Another advantage of the test is the coarse size 2 – 3 inches which makes it unique in the series of tests.

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Grindability tests

SAG power index (SPI)

• SPI expressed in minutes , is the time T necessary to reduce the ore from P80 of 12.5mm to P80 of 1.7 mm• The SPI has the

advantage of requiring low weight and is suited for geometallurgical mapping of ore deposits

• SPI is widely used and deposits can be compared in terms of hardness and variability, see fig below.

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Grindability Tests

JKTECH drop weight test

• Developed by JKMRC• Divided into 3

components:• Test measures the

resistance to impact breakage of coarse particles in the range 63 – 13.2 mm

• Then evaluates the resistance to abrasion breakage in the range 53 – 37.5 mm

• Finally the rock density of 20 particles is measured to asses the average ore density as well as its dispersion.

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Grindability test


• The test generates the appearance function –• E.g. the breakage pattern of

the ore under a range of impact and abrasion breakage conditions

• The appearance function can be used in the JKSimMet modelling and simulation package to predict the ore response to comminution process

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Grindability tests

JKTECH Drop weight test

• Also part of these procedure is the density determination of 20 rock samples, using water displacement techniques.

• Figure 5 shows an ore displaying a wide range of densities.

• The density distribution of the ore is important in AG/SAG milling because

• It affects the bulk density of the charge and associated power draw

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Grindability tests


• A great number of rock weight tests have been performed over the years which allows for comparison of ore types in a data base.

• The frequency distribution of the function ‘A x b’ from JKTech is depicted in Fig 6

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Grindability Tests


• One of the interesting features of the drop weight test procedure is that it provides a variation in rock hardness by size from 13.2 to 63 mm.• Fig 7 illustrates this at 3

different energy levels.• 0.25 1.0 and 2.5 kWh/t

• For a very competent ore, the curve will be nearly horizontal, a non-competent fractured ore will show a high gradient with increasing size

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Grindability Tests

SAG Mill comminution test

• This is an abbreviated drop weight test, which can be performed at low cost on small samples 19 – 22 mm or drill cores.• 5 kg of sample is

normally sufficient.

• The advantage of the SMC test is that it generates the energy versus breakage relationship with as small quantity of sample of a single size fraction.

• Because the test can be performed on small rocks, it is well suited for geometallurgical mapping.

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Grindability testsMacPherson Autogenous

Grindability tests

• This is a continuous test performed in a 46 cm semi-autogenous mill with an 8% ball charge.

• The pilot plant consists of a feed hopper, cyclone, screen and dust collector with a control system to regulate the charge volume and circulating load.

• 100 to 175 kg of sample is required with a top size greater than 25 mm.

• The test is run continuously for 6 hours.

• The importance of reaching a steady state in a grinding mill is widely accepted, this test is the only small scale test that offers the option. 17

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Grindability TestsMacPhersons Autogenous

grinding tests

• Throughput rates • Specific Energy

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Grindability Test

Media Competency test

• There has been some variations of media competency test developed over the years with the assessment of media survival in autogenous milling being the main objective.

• 104 to 165 mm rocks are subjected to a tumble test using 10 large rock in 5 size fractions.

• The surviving rocks are submitted to fracture energy test procedure.

• This provides the relationship between the first fracture energy requirement and rock size. 19

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Grindability tests

High Pressure Grinding Rolls

• HPGR are emerging as an energy efficient alternative to AG/SAG circuits.

• The traditional method for testing is processing large samples in a pilot scale.

• Several tests are performed to asses the effect of operating pressure and moisture content on HPGR performance

• The power input is recorded and presented below.

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Crushability test

Impact Crushability

• Gives a WI that can be applied to 3 types of crushers

• Gyratory – WI can be used to determine the horse power.

• Impactors – WI is an indication of hardness

• Cone Crusher – rate the material to determine the duty of the crusher 21

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Crushability Tests

Paddle Abrasion

• Results are in the form of Abrasion Index and chemical makeup of the material

• Tests are used to determine whether an Impactor or cone crusher is suitable.

• Can also be used to calculate the approximate liner life for the crusher 22

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Crushability tests

French Abrasion

• Gives an Abrasion and Crushability Index• Mainly used to estimate

hammer wear in the Impactor application

Dynamic Fragmentation

• Conducted for Impactor application• Measures the friability of

the material• Dynamic fragmentation

number will indicate if the Impactor is feasible for a particular application.

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Discussion Points!

• Where can I apply Bench scale and pilot scale programs in my work environment?

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FACTORS INFLUENCING THE SELECTION OF COMMINUTION CIRCUITS

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Fact

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• Geological Interpretation of Drill core and Bulb Sample• Mineralogical

Analysis• Chemical Analysis• Physical Properties• Circuit feed

Parameters

• Sampling requirements• Contiguous

properties• Feed and product

Specification• Bond work Indices,

Abrasion Index, and specific power consumptions 26


• Circuit selection• Metallurgical

efficiency• Cost Consideration

• Water supply• Fine Grinding• Plant layout

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Geological Interpretation of Drill core and Bulk Sample

Information Gained

• Identification and relative abundance of Mineral content

• Degree of Dissemination• Type of Lithology• Types of Alteration• Degree of Oxidation• Geotechnical

Competence• Hardness

Effect on Circuit Selection

• Provides a guide to the types of circuit required and the types of samples required based on precedent

• Determines the necessity of separate plants to process sulphide ores

• Provides a guide to the selection of autogenous grinding 28

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Mineralogical Analysis

Information Gained

• Identification of ore and gangue minerals and middling association• Liberation and Modal

Analysis• Quantitative analysis –

QemScan


• Determine Ratios of reduction• Feed and product size

analysis in primary , secondary and regrind circuits

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Chemical Analysis

Information Gained

• Identification of metallic , non-metallic and acid generating constituents


• Determining the requirements of pre-washing the ore

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Physical Properties

Information Gained

• Hardness, Blockiness, Friability, Quantification of primary fines and clay content• Specific gravity of

mineral constituents


• Provides a guide to potential problems in Crushing Screening and Grinding the ore with respect to equipment selection and Over grinding and avoidance of slimes generation with respect to softer minerals. 31

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Circuit feed Parameters

Information Gained

• ROM top size parameters• Primary crusher

discharge size analysis• Throughput

requirements and schedules• Mining Plans , Schedules,

methods and equipment sizes


• Determines selection of primary crushers and necessity for pre-crushing can influence this selection by determination of the product size at the required throughput rate.

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Sampling requirements

Information Gained

• Preliminary drill core for resource definition and split for bond work indices

• Whole core for Autogenous Media Competency Index, Impact crusher work indices and fracture frequency

• Bulk Sample , large diameter drill core, open pit or underground for pilot plant testing


• Preliminary Assessment of grinding requirements and ore variability

• Power based methods for mill sizing using results from Bond , Impact and grinding work indices

• Assist in definition of Pilot plant test program and ore Variability Characteristics

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Contiguous properties

Information Gained

• Definition of equipment characteristics


• Determines the utility of equipment with respect to its Inherent operating behaviour, e.g. Autogenous grinding mills grinding to a natural grain size, SAG mills breaking across grain boundaries and rod mill minimizing the creation of fines 34

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Feed and product Specification

Information Gained

• Definition of requirements at each comminution stage


• Influence of “Mine to Mill” and choke feeding the primary crusher on subsequent stages Performance• Maximum feed top size

in relation to high aspect and low aspect primary mills• Use of HPGR 35

Fact

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Bond work Indices, Abrasion Index, and specific power consumptions

Information Gained

• Calculation of specific power consumption at each comminution stage for different ore types and composites.

• Assessment of ore variability• Checking on pilot plant test

data• Assessment of risk or

contingency based on samples selected according to the mine plan


• Distribution of power Confirmation of specific power consumption and contingencies for Process design criteria

• Calculation of estimates for media and liner wear.

• Estimation of mill power requirements and distribution of power between equipment

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Circuit selection

Information Gained

• Assessment of Overall Power requirements and power efficiency for different circuit options

• Assessment of Overall Operating Availability for different circuit options

• Determination of unit power cost and demand for different circuit options


• Determination of the Most economic option on the basis of NPV of Capital and Operating cost and circuit availability for a fixed revenue rate.

• Power efficiency should be optimised in design for each circuit option considered.

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Metallurgical efficiency

Information Gained

• Definition of Optimum comminution configuration• Definition of feed rate

variation• Selection of grinding

media


• Determination of necessity for stage grinding and stage concentration to optimise mineral liberation and recovery.

• Quantify the effect of feed rate variations on the metallurgical efficiency of down stream processes.

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Cost Consideration

Information Gained

• Definition of Largest practical equipment size and design• Differences between

comminution options


• Effect of efficiency on crushing and grinding equipment E.g. Separation of screening plant from crushing plant.• Feed arrangement

requirements• Choke feeding crushers

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Water supply

Information Gained

• Definition of Process alternatives


• Determination of plant location Namely, Mine location, Applicability of dry grinding, Pre-concentration and use of sea water.

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Fine Grinding

Information Gained

• Determination of test requirements, batch and / or Pilot scale tests


• Determination of Optimum location of Fine grinding application within the circuit and definition of the types of machines used.

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Plant layout

Information Gained

• Definition of Geographic location, Climatic conditions, Accessibility• Definition of relative

location of Mine vs. Plant• Definition of Operating

schedules and manpower requirements• Definition of expansion

potential


• Determination of wet and dry processes

• Determination of Physical sizes of equipment and foot print of the plant

• Determination of built-in contingencies that allow for future expansion

• Consideration for the addition of equipment lines in the case of larger plants. 42

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Discussion Points!

• Comments on pertinent factors that was involved in the selection of your plant system.• The pros and cons of the

current system, bottle necks, etc.

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TYPES AND CHARACTERISATION OF CRUSHER EQUIPMENT AND CIRCUIT FLOWSHEET

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Type

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Introduction

Standard Equipment

• Crushing flowsheet and equipment are selected to prepare ore for downstream purposes. Standard equipment for the minerals industry has been :

Jaw crushersGyratory crushersCone crushers

New Equipment

Water flush cone crushers

Vertical and horizontal impactors

High pressure grinding rolls

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Factors Affecting crusher selection

• Plant throughput, ore delivery schedules• Size of feed• Desired product size for

down stream processing

Ore characteristics:Hard rock ClayGravelVariability

Climatic conditionsDown stream processes

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Type

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Plant throughput and ore delivery schedules

• Forms the base line for flowsheet design and equipment selection• Size type, number of

stages and number of crushers per stage for an application can be identified.

• E.g. A primary Jaw crusher will be better suited for a conventional underground mining operation because:

Tonnages are typically lower

Feed material size is smallerLess headroom and a

smaller excavation is required.

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Type

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Feed size

• The crusher selected must be sized for throughput as well as top size expected from the mine.• Smaller the crusher the

smaller the dimension of the feed material that can enter the crusher chamber.

• A balance between the plant capacity and the size of the crusher must be reached.

• In multi stage crushing circuits the products of the preceding stage will be the determining factor in the selection of the size of the crusher and the crusher liner configuration. 48

Type

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Product size

• The target product size required from the crushing circuit will determine the number of crushing stages and types of crushers to be used for a specific application.

• E.g.. To produce a coarse product a single stage crusher may be required.

• To produce a 15 mm product a two stage crushing may be required.

• The ability to crush finer has been required for specific application.

• For fine product sizes in dry process application flowsheet have incorporated vertical shaft impact crushers operated in closed circuit with vibrating screens. 49

Type

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Ore Characteristics

• When selecting equipment for inclusion in a crushing flowsheet the following factors should be considered:• Hardness• Toughness• Abrasiveness• Moisture content• mineralisation

• Geologists should provide info with regards to:

• Rock types• Abundance of various

rock types LOM• Short and long term

delivery schedules should then be provided mining to adapt circuit configuration for LOM 50

Climatic Conditions

• A dry warm climate will allow for an unenclosed installation.• Colder wet climates will

require enclosures for operator protection and moisture problems.

• An enclosed crushing plant also posed dust extraction challenges.

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Type

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Downstream processes

• Heap Leaching• Crusher product size will

be specified for optimum recovery

• Milling• Type of grinding circuit will

influence the number of crushing stages.

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Type

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Application

Primary Crusher

• Purpose• To reduce the ore to a

size amenable to secondary crushing, SAG mill feed or heap leach product• Usually operated in open

circuit.

• Typical crushers used are• Jaw• Gyratory• Horizontal impactors • Rotary breakers• Ratio of reduction 8:1• Some form of scalping

screen may be installed in the case of Jaw and Impact crushers 53

Type

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Application

Secondary Crushers

• Purpose• To produce an

intermediate or final product

• Feed Size – typically between 200 & 75 mm depending on primary crusher

• Vibrating screen may be installed ahead to remove product size material.

• Crusher types:• Standard cone crusher –

traditionally• Horizontal Impact

crusher as alternative• HPGR recently for

diamond and iron ore

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Type

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Application

Tertiary Crushers

• Purpose: Produce the final product• Feed : 37 mm• Product : 12 mm

• Crusher type:• Short head cone crusher• Longer crusher chamber

and more even size distribution

• Usually operated in closed circuit with a vibrating screen

• HPGR and Nordberg Water Flush crushers have also been used.

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Type

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Application

Quaternary Crushing

• Purpose:• To produce fine dry

product for downstream processing

• Vertical Impact Crusher has been used at Newmont’s heap leaching operation in Uzbekistan.

• High speed crusher that used high speed impact to effect particle reduction

• Nordberg’s Gyradisc crusher uses a combination of impact and attrition to effect particle size reduction. • Applied in the industrial

minerals and sand industry to produce finished products to 800 microns. 56

Type

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Crusher types

• Jaw• Gyratory• Horizontal shaft impact

crushers• Rotary breakers• Roll Crushers

• Cone crushers• Gyradisc crushers• Vertical impact crusher

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Type

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Flowsheet – Two stage Crushing (Fine Product)

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Two Stage Crushing (Coarse product)

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Three Stage Crushing

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Two Stage with Water flush Crusher

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Three Stage Crushing – Gold Heap Leach

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Three stage crushing and Water flush Crusher

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Water flush Crushing

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SABC configuration

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Three Stage Crushing with Vertical Shaft Impactors

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Discussion Points!• What are the Problem areas of current equipment

installation?

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Type

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Crus

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SELECTION AND SIZING OF PRIMARY CRUSHER

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Sele

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ush

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Introduction

• The rock / ore determines the type of crusher• The plant capacity

determines the size of crusher

Family of primary crushers

• Gyratory• Double toggle Jaw• Single toggle Jaw• High speed roll crusher• Low speed sizer• Impactors• Hammer mill• Feeder breaker

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History

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Mechanical Reduction MethodsFour basic ways to reduce a

material

• Impact • Attrition• Shear• Compression

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Sele

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Compression

• Done between two surfaces• Gyratory and double

toggle jaw uses this method

Should be used when

• Material is hard and tough• Material is abrasive• Material is not sticky• Uniform product with a

minimum of fines is desired• The finished product is

relatively coarse > 38 mm• Material will break cubically

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Sele

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Impact

• Refers to sharp , instantaneous impingement of one moving object against another• Two types• Gravity• Dynamic

Conditions

• Cubical particles are needed• Finished product must

be well graded• Ore must be broken

along natural cleavage lines• When material is too

hard and abrasive or high moisture content 75

Sele

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Attrition

• Scrubbing material between two hard surfaces• Hammer mills operate

with close clearance between hammers and screen bars and reduce by attrition combined with shear and impact reduction.

Conditions

• When material is friable and non-abrasive• When top size control is

not desired• When maximum of fines

is required.

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Shear

• Consists of trimming or cleaving action

• Exploits the fact that the ratio of compressive strength to tensile and shear strength in the majority or rocks is approximately 10 : 1

• Low speed sizers break the rock in tension and shear by chopping action

Conditions

• When the material is somewhat friable and has low silica content

• When material is soft to medium hardness

• For primary crushing with a reduction ratio of 6 : 1

• When a minimum of fines is desired

• When a relative coarse product is desired > 38 mm 77

Sele

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Primary Gyratory crushers

• The main capacity advantage offered is centred around the Archimedes principal

• They found that the crushing chamber provides more effective volume than a rectangular volume• The shaft grating speed

adds a third dimension to crushing as opposed to two dimensional crushing 78

Sele

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Gyratory crusher

Advantages

• Designed for direct dump from trucks Lowest maintenance per ton processed of any designed crusher• Can handle crushing ore

hardness up to 600 mPa• Easy handling of tramp

material with hydraulic reiief system

Disadvantage

• Highest installed capital cost of any crusher design

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Sele

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JAW CRUSHER ANIMATION VIDEO 1

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WORKING PRINCIPLES OF THE JAW CRUSHER VIDEO 2

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Double toggle design

• The swing Jaw of the Standard DT crusher pivots from an overhead shaft .

• A Pitman hung from an eccentric shaft transmits motion through a pair of toggles at the bottom of the swing Jaw

• Swing Jaw motion is greatest at the discharge opening.

• The hinge pin is located behind the centreline of the crusher zone and it causes the swing Jaw to move perpendicular to the fixed Jaw. • This arrangement

provides twice the force in crushing • Typical duty is 350 MPa 82

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Double toggle Jaw

Advantages

• Lower installed cost than a Gyratory crusher• Can handle high abrasion

with low maintenance• Can handle tough

crushing application upto 600 MPa nickel ores, iron ores, etc.

Disadvantages

• Same capacity limitations as the single toggle aw crusher• Substantially higher

installed cost than a single toggle Jaw crusher• Same crushing size

limitation as single toggle Jaw crusher

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Single toggle Jaw crusher

• The rotation of the eccentric shaft causes the swing Jaw assembly to move in an elliptical path.

• Maximum movement of the swing jaw assembly occurs at the top of the crushing chamber with minimum movement at the discharge opening

• At all points in the crushing chamber the crushing action has both vertical and horizontal components.

• Due to the rubbing action of this type of jaw, jaw plate wear is accelerated and power efficiency is lowered because the swing jaw is lifted on every stroke.

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Single toggle Jaw crusher

Advantages

• Lower installed cost than a double toggle• Lower power usage than

a double toggle• Can handle sticky,

muddy ore easier than a double toggle or Gyratory

Disadvantages

• Normal economic maximum capacity is 750 MTPH

• Duty of crusher is for light or medium hard material

• Does not handle high abrasive material as well as DT

• Requires feeder• Primary crushing only 85

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Low speed sizers

• The low speed sizing principle is the combination of high torque / low roll speeds.• The interaction of tooth,

spacer and roll set up a “sized void” which in turn sizes the material

• Used for non-abrasive sticky type material bet 200 - 400 MPa• Application• Medium hard limestone,

bauxite, kimberlite, gypsum, clay, shale and gold ore.

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Low speed sizers

Advantages

• Can handle high tonnages – 12 000 MTPH• Low installation cost and

minimum head room required• Low fines production• Low power consumption• Easy rejection of oversize

feed – using discharge gates

• Low reduction ratio• Peak power loading up

to 8 times installed power• Not economic for low

tonnage unless the material is very difficult to handle

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Single toggle vs. Double toggle

• ST has a larger angle of nip, the larger the nip angle the harder to grip the material..• ST – greatest movement

at the top• DT – greatest movement

at the bottom

• ST – Movement of jaw is in downward rolling direction which gives a force feed action assists in handling sticky material • Life of Jaw in ST is less

than DT

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Impact crushers

• Utilized in soft, non-abrasive application• Crushing availability and

maintenance can economically offset against capital cost

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OPERATION OF AN IMPACT CRUSHER VIDEO 3

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IMPACTOR ANIMATION VIDEO 4

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Impact Crusher

Advantages

• Can handle larger size reduction 1000 : 75• High reduction ratio

compared to investment cost• Provides a high degree of

fines• Can handle up to 2500

MTPH

Disadvantages

• Requires feeder• Cannot handle tramp

metal• Higher power

consumption as more fines are produced• High wear due to higher

silica content + 8%

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Feeder Breakers

• Are utilised in soft to medium hard application• Coarsely break material

for belt conveying• Frequently used for

overburden and underground duty

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Feeder Breaker

Advantages

• Avoids costly site preparation and civil work

• Can transfer and crush material in a single machine

• Handles wet material with ease

• Very low headroom• Can handle upto 2000

MTPH

Disadvantage

• Very low reduction ratio• Crushing takes place in

breaker bars and chains which causes wear.

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Primary Crusher selection Criteria

• Will it produce the desired product size at required capacity

• Will it accept the largest feed size expected

• What is the capacity to handle peak loads

• Will it choke or plug• Is the crusher suited to the

type of crushing plant design

• Is the crusher suited for underground or in-pit duty

• Can it handle tramp material without damage

• How much supervision is required

• How does the crusher resist abrasive wear

• What is the power consumption 95

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Primary Crusher selection Criteria

• Does the crusher operate economically with minimum maintenance• Does the crusher have

an acceptable parts replacement cost• Does the crusher have

easy access to internal parts

• How does the initial cost of the machine compare to the long term operating cost.

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Primary crusher selection - Capacity

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Primary Crusher Selection – Feed Size

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Primary Crusher selection – Product size

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Primary Crusher Selection – Compressive Strength

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Primary Crusher Selection – Abrasion Index

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Primary crusher selection – Clay content

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Primary Crusher Selection – Underground Application

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Primary Crusher Selection – Mobile Plants

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COMPUTER AIDED DESIGN OF JAW CRUSHER

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Components of a Jaw Crusher

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Material for components of Jaw Crusher

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Kinematic Analysis of Jaw Crusher• The geometry of the

moving Jaw results in a movement change which has a great effect on the crushing action and particle breakage.

• Based on the analysis of the moving jaw movement, the squeezing process and the crushing force distribution, the jaw plate wear on a macroscopic scale level aiming to predict the wear distribution on the jaw plate can be studied. 108

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Swinging Jaw movement• The reciprocating jaw

MN driven by the eccentric shaft AB does kind of a periodic plane swing movement.• Jaw crusher can be

considered as a four bar mechanism in which link AN is the crank and OA is the fixed link 109

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• MN is the moving jaw and OM is the toggle bar.• In the analysis we are

intended to find out the displacement, velocity and acceleration of various points on the swinging jaw plate. 110

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Data extracted from standard Jaw crusher• Length AN = 172 cm• Length MN = 1085

cm• Length OM = 455 cm• Co-ordinates of A

(45.3 , 815.7)• Crank angle rotates

from 0 to 360 degrees anticlockwise. 111

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Crank angle vs. angle made by moving jaw

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Crank angle vs. Angle between moving Jaw and Y axis

• The graph shows as the moving Jaw approached its counterpart which is stationary it tends to be vertical i.e. the angle between the moving Jaw and the Y axis decreases as a result the crushed product slips downwards.

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Vertical Displacement Vs. Horizontal displacement

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Horizontal displacement Vs. Crank angle

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Displacement Vs. crank angle

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Points on the moving Jaw• Every point on the

moving Jaw follows an elliptical path• When it moves

towards the fixed Jaw, it goes vertically down and in the return stroke it moves vertically up.

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Vertical Velocity Vs. Crank angle

• The rate of change of vertical velocity is greater for the topmost point and decreases downwards

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Horizontal Velocity Vs. Crank angle

• The rate of change of horizontal velocity is greater for the bottom most point and decreases upwards

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Velocity Vs. Crank angle• The maximum rate of

change of final velocity is greater for the points away from the crank.

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Horizontal acceleration Vs. Crank angle

• With progress from 0 to 360 degrees crank angle rotation the horizontal acceleration first increases then decreases

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Vertical Acceleration Vs. Crank Angle

• With progress from 0 to 360 degrees crank rotation the vertical acceleration first decrease then increases

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Acceleration Vs. Crank Angle• The maximum

acceleration is observed for the points farthest away from the crank angle

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Effect of sliding motion on Jaw wear• Breakage Analysis• 3 types of Fracture

mechanisms are observed• Abrasion• Cleavage• Shatter

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Breakage Analysis• The particle fracture

mechanism in the Jaw crusher chamber is a mixture of cleavage and abrasion. The abrasion fracture is caused with the localised too much energy input to the area directly under the loading points and the

• Friction between the Jaw plates and the particle.• The induced tensile

stress results in the cleavage fracture.

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Crushing Process• Theoretically a

particle inside the crusher is crushed when it is compressed and fails in tensile stress.• In practice the

particles also undergo slipping motion between the jaw plates

• The forces acting on the element during the crushing process is shown below

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Crushing Process• As the horizontal and

vertical velocities of the moving jaw changes during the crushing process, the forces on the particle varies at different times.

• When the component of the vertical velocity is greater than the components of the horizontal velocity the forces on the particle is shown in Fig. 3.3 (a) 127

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Crushing process• When the

component of the vertical velocity is less than the components of the horizontal velocity the forces are shown in Fig. 3.3 (b)

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Crushing process• By a resolution of forces

acting on the particle as shown in figure 3.3. it can be proved that conditions for the particle to slip against the fixed jaw plate is much greater than with the moving jaw plate. Condition for slide between the particle and the fixed jaw plate is unavoidable

The chance for the particle to slide is greater with the fixed jaw than the moving jaw.Due to vertical motion irregular geometry of particles, a classification process before the particle fracture may exist during close process in which the particle adjustment may take place. 129

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Wear Analysis• Squeezing and sliding are

the two principal factors affecting the Jaw plates wear

• Squeezing plays the main role at the top of the crusher and the wear is small.

• As the particles move down the crusher the probability of slip increases and the wear becomes more pronounced.

•

• At the middle lower part of the crusher where the ratio of the vertical distance to the horizontal stroke reaches a maximum value resulting in maximum wear of the crusher.

• The slide between the fixed Jaw and particle is greater compared to the moving jaw hence the wear is dominant in the fixed jaw.

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Discussion Points!• What are the flaws of

the current primary crusher installation?• Where can we

improve?

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SELECTION AND SIZING OF SECONDARY AND TERTIARY CRUSHERS

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Introduction

• Modern crushers have increased in performance• Evolved to focus greater

on the quality of desired product• More stringent

requirements are being placed in terms of shape and gradation.

• Proper size reduction results in better recoveries

• In milling feed preparation, the generation of fines and total top size reduction results in maximum mill productivity.

• Proper understanding of crusher capabilities will minimize both installation and operating capabilities. 133

HOW THE SYMONS CONE CRUSHER WORKS VIDEO 5

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NEW GENERATION OF CONE CRUSHERS VIDEO 6

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Cone Crushers

Modern Cone crushers

• Increased performance capabilities• More power capabilities• Larger in size• Higher capacities• Better product shape• Higher percentage of

final product yield

New cone crushers

• Safer more reliable hydraulic clamp and clearing system to protect the crusher from uncrushables and overload conditions

• Adaptation of hydraulic setting adjustment system in the cone crusher design improves overall efficiency of crushing operation 136

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New cone crushers

• New generation of cone crushers provide • ease of operation• Simple maintenance• Uniform production

throughout the liner life• High availability

• Technology has evolved to include computer controls to maximize and optimize crusher performance based on application requirements

• Modern devises provide real time feedback :

• Power draw, cavity level, crushing force, temperatures, pressures, etc. 137

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Cone crusher selection criteria

Information required

• Capacity required with consideration for expected availability• Expected gradation and

product size

Material characteristics

• Specific gravity• Bulk density• Impact work index• Moisture content• Abrasion index• How the material breaks• Small scale lab tests and

full scale pilot tests

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Cone crusher design limits

Design limits

• Volume limits• Power limits• Force Limits

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Design Limits - Volume

• Maximum rate of feed to the cone crusher without overfilling the cone crusher feed hopper• Function of • Speed of the crusher• Closed side setting CSS• Head angle• Material density

Defining variables

• Feed gradation• Crusher chamber

configuration• Transport of material

through the crusher cavity• Fragmentation

characteristics

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Design Limit - Power

• Power limit is reached when average power draw kW exceeds the installed motor power of the crusher.

• Ore of high impact work index or strong resistance to fragmentation tend to reach or exceed the power limit easily.

• Pilot scale test work can provide information regarding power consumption

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Design Limit – Force factor

• The force limit of a crusher is reached when the combined forces exerted during crushing exceeds the force available on the machine to hold the desired closed side setting.

• Force limits may be exceeded due to

• uncrushables material entering the crushing chamber

• Operating at a small closed side setting

• Packing of wet sticky material• High power draws• Incorrect crushing cavity

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Cone crusher sizes and Capacity ranges

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Secondary Cone Crusher Selection

• Ensure the feed material does not exceed the acceptable maximum size for the crusher• Determine the capacity

requirements at a given closed side setting based on a 4/6:1 reduction ratio.

Example

• maximum feed material 200mm• Capacity 500 tph• Table 1 : HP 300• At 32 mm CSS the

crusher is unable to achieve a minimum of 500 tph• Table 1 : HP500

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Secondary Cone Crusher Selection

Correct cavity configuration

• The cavity configuration has to suit the feed gradation so that the maximum crushing performance and liner utilisation is achieved

• Several cavity configurations are available for cone crushers to maximise performance.

• An improper liner configuration applied can create high crushing forces leading to adjustment ring movement , exceeding crusher force limit.

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Case Study: HP700 replacing Hammer mill

Copper mine in Portland

• Hammer mill used to prepare rod mill feed• Hammer mill replaced by

HP700 cone crusher

Results

• 20% gain in energy efficiency• By reducing the rod mill

feed from 80% passing 30 mm to 80% passing 14 mm.

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The Pre-crusher option

• The most recent evolution for pebble crushing finds a basis in the presumption that the most appropriate primary mill feed contains a minimum amount of critical size material.

• The initial feed of the primary mill should dominantly consist of fine and coarse material.

• Coarse material serves as impact media and fines as transport medium for down stream processing.• Pre-crushing targets to

convert the middling to fine fraction.

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Case Study: Pre- Crushing

Troilus Mine

• 150 – 50 mm is pre-crushed using an HP 700 cone crusher• Production increase and

operating cost decreased.

Kidston Mine

• All primary crusher ore is pre -screened to remove fines

• All +50 mm oversize is crushed at maximum reduction ratio to deliver maximum fines.

• Proved effective in boosting milling productivity and lowering operating cost.

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Typical Pre-Crusher installation

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TRACK MOUNTED CONE CRUSHER VIDEO 7

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CASE STUDY : INFLUENCE OF ECCENTRIC SPEED OF CONE CRUSHER PRODUCTION AND OPERATION

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Case Study : Pilot test program

• The research was performed in Tampere, Finland using an HP 200 cone crusher• The study can be

separated into three groups of test:• Base tests• Fixed tonnage tests• Feed size distribution tests

• The base tests were used to measure the crushers maximum performance for a given eccentric speed.

• The fixed tonnage tests simulated operating conditions where the feed rate to the crusher is limited below the maximum capacity based on the base eccentric speed and CSS 154

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Case Study : Pilot Test Program

• A third set of tests utilized a different feed size in order to verify results as well as reducing the effect of top size particles possibly being inhibited to enter the crushing cavity.

• The tests in each group used the same homogenous feed of known characteristics with feed sample being taken every forth test for verification.

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Case Study: Pilot test results

Overall

• Most of the data showed clear trends in capacity, power and discharge size distribution as the eccentric speed was varied.

Base testing results

• For the base testing where each test was operated at the optimal cavity level to develop a baseline for maximum production, the results matched theory.

• As the eccentric speed was increased the capacity decreased in a nearly liner manner.

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Case Study : Base Testing Results

• On average, the total capacity tph fluctuated by 22.5% over a design speed range of 34%.

• The increase in capacity but decrease in reduction as the speed is lowered results in relatively low changes to power draw as shown in figure 2.

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Case Study: Base Testing Results

• For a base case testing with a full cavity throughout, it was seen that there was slight benefits in throughput and energy efficiency when the crusher was operated at near the minimum design eccentric speed.

• The higher capacity outweighed the slight loss in reduction through the machine and the machine was more mechanically efficient at the lower speeds. • It was best to operate at

the low end of the speed range. 158

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Case Study: 32 mm CSS Production Vs. Specific energy

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Case Study : 19 mm CSS Production Vs. Specific Energy

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Case Study: Fixed Tonnage Test Results

• The tests operated at a fixed tonnage were conducted to simulate a crushing application where the crusher is not the limiting equipment therefore the tonnage to the crusher is fixed by other plant limitations therefore the crusher cannot normally achieve a full choke condition.

• The power draw of the crusher dropped significantly as the speed decreased resulting in a lower kW/t specific energy through the machine.• There was a major shift

in reduction through the machine ‘ 161

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Case Study: Fixed tonnage test results

• The tph of the -12.5 mm product fell slightly as the eccentric speed reduced from the reference speed by 20%

• The phenomenon occurred at the point where the cavity level in the crusher could not fill up half of the crushing chamber and the discharge became coarser

• While operating with a higher cavity level was more efficient, the crusher was more mechanically efficient at the lower speeds 162

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Case Study : Fixed tonnage test results

• For the fixed tonnage tests there was a marked improvement in the variation of power draw as the speed and cavity level increased.

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Case Study : Practical Application

• There are a number of uses for these principles in a crushing plant. The main points are as follows:

• Changing the speed to find a more optimal setup than that supplied by the manufacturer.

• Manipulating the speed based on current static plant conditions,

• And dynamic control of eccentric speed in a control system.

• The optimization of eccentric speed may be beneficial where feed conditions and plant requirements change.

• Dynamically manipulating the eccentric speed using a variable frequency drive has not been widely used.

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Case Study : Practical Application

• A dynamic control system can be used to vary the speed resulting in benefits to production and energy efficiency.

• E.g. When the throughput of the crusher is high it could be operated most efficiently in the lower speed range.

• If the throughput requirements drop for a short period of time it would be more productive and efficient to increase the speed of the crusher and operate with a fuller chamber.

• An underlining benefit for greater control of the crusher operation is maintaining a choke fed condition, which has benefit to production , operating cost and mechanical health of the crusher. 165

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Discussion Points!

• Choke feeding in your current application, the pros and cons.

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SELECTION AND SIZING OF HIGH PRESSURE GRINDING ROLL CRUSHERS

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HPG

R

NEW CRUSHERS ON THE MARKET VIDEO 8

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HPGR Introduction

• HPGR are well established in the cement industry for the grinding of clinker, limestone, slag and other relatively non-abrasive material.

• Minerals are 20 – 100 times more abrasive than cement raw materials.

• Acceptance by the minerals industry has required the development of special wear protection surfaces and rapid change out procedures for the rolls.

• Range of grinding• Coarse < 75 mm • To grinding of fine

concentrate < 100 microns 169

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HPGR Introduction

• Moisture content up to 12 %• Machines are available

with capacities up to 3000 tph• Installed power up to

6000 kW

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HPGR Installed in Diamond and Iron Ore Industries

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HPGR – L/D ratios

Length to Diameter ratio

• Is it more advantageous to design rolls with smaller diameters and larger widths or larger diameters and smaller widths?

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HPGR – L/D Ratio

• The decision as to which approach to adopt is capital.

• It has an impact not only on the performance of the crusher but also major impact on the design of the individual components and on the general layout of the unit.

• The minimum roll diameter is prescribed by the outside diameter of the bearings and the thickness of the bearing block.• The bearings are sized

according to the installed grinding force.

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HPGR – L/D Ratio

• The size of the bearing determines the shaft diameter and pre-determines the manner in which the gear box and shaft are to be connected.

• Larger rolls with low L/D ratios offer greater freedom in selecting the most appropriate bearings.

• The larger roll diameter makes the connection between the shaft and the gear box simple to execute. And allow large gear boxes to be located on one side to save space and facilitate maintenance. 174

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HPGR – Roll design

• Three different roll designs have been successfully applied:• Solid rolls• Rolls with tyres• Rolls with segmented

liners

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HPGR – Criteria for selecting optimum design

• The balance between operating and investment cost• The acceptable lifetime

and frequency of replacement• The tolerable down time

for liner replacement

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HPGR - Comparison

Tyres

• Lower investment cost• No interfaces (joints)

• Longer lifetime• Lower wear cost• No pressure restriction

Segments

• Higher investment cost• Joints between segments

require more maintenance due to washouts• Shorter lifetime• Higher wear cost• Only for low pressure

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HPGR - Wear protection surfaces

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HPGR - Wear protection of roll surfaces

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HPGR – Key Parameters

• Achieve the throughput requirements and to achieve the desired product fineness

Throughput

• Function of roll dimension• Type of roll surface• Feed material properties

• For a given material and roll dimension the throughput is controlled by the roll speed.

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HPGR – Key Parameters

Product Fineness

• Controlled by the grinding force applied to the material bed between the rolls.• The grinding force

creates the pressure in the material bed which causes micro-cracks and breakage of the particles.

• The correlation between particle breakage and grinding force required needs to be determined for each material

• Key parameters are • Specific throughput rate• Specific press force to be

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HPGR – Throughput rate vs. roll speed

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HPGR – Feed moisture content vs. Throughput rate

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HPGR – Throughput vs. Size distribution

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HPGR – Product fineness

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HPGR – Product of various ores

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HPGR – Energy consumption vs. Force

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HPGR – Energy Input vs. Roll Surface

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HPGR - Energy Input for various ores

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HPGR – Energy input vs. Grinding force

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HPGR Wear Factors

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HPGR – Roll Diameter Vs. Roll Speed

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HPGR - Application

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HPGR – Pebble Crusher

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HPGR – Pre-Crusher

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HPGR – Replacement of 3rd and 4th stage

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Discussion Points!

• Is it possible to include HPGR in your circuit?

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ORE CHARACTERISATION

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Characterisation - Understanding the ore body and the Metallurgy

• The best possible characterisation of the ore body will enhance the ability to extract better outcomes from a mine to mill application.• The greater data, the

better characterisation of the ore body. Properties.

• This characterisation is important in developing extraction and processing strategies which enhance the productivity gains possible from a mine to mill application (JKMRC 1998)

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• At its simplest , characterisation is about developing the best possible understanding of the ore body , in particular its variability.

• One of the first comprehensive characterisation studies was reported by Simkus and Dance (1998) at the Highland Valley Mine

Highlands Valley• Had developed a program

mapping the hardness of different ore types, since the late 1970’s.

• By late 1990’s , drill monitors were being used to provide an estimate of ore hardness of subsequent blasted ore.

• Ore was then tracked to stockpiles using mine dispatch systems and movement through stockpiles was modelled. 200

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• An image analysis system was used to provide an estimation of the feed size distribution to the SAG mills.• Relationships were

developed between ore hardness, feed size and mill throughput.

• This approach provided a strong ability to predict expected mill throughput information which could then be utilised in process control.

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Rock Mass Properties

• Standard rock mass properties are usually obtained as geotechnical information from drill core and include:

• Rock Mass Rating• Rock quality designation• Point load Index• Young’s Modulus• Poisson’s Ratio• Unconfined Compressive

stress• In-situ block size• Joint spacing 202

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Metallurgical Process Parameters

• These data typically include:

• Grades, including the grades of gangue minerals and minor elements

• Grindability data, principally related to ore hardness, as measured by bond work indices and JKMRC grinding model parameters,

• Flotation grade and recovery data as determined by laboratory flotation tests• Mineral liberation• Lithology• Geological Alteration• Acid forming potential of

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Predictive Models

• Models frequently used in mine to mill studies include

• Mine block models incorporating geotechnical and geometallurgical parameters.

• Blast fragmentation models• Muck pile models• Comminution models

• Models which predict the final stockpile shape resulting from open pit blast are increasingly useful when it is desirable to understand where material of different properties, notably grade, reside in the muck pile after blast. 204

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Conclusions

• The literature analysis suggests that the tools required to implement Mine to mill approach are available in acceptable form.

• Many of these hardware and software tools are provided by established suppliers and have been successfully implemented.

• Most tools are also subjected to research and further development

• The area of greatest need is the availability of tools to monitor mine to mill outcomes.

• To date these have been developed at individual sites

• More generic software tools would be useful. 205

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CASE STUDY: ANTAMINA BOOSTS THROUGHPUT FOR HARD ORES

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Case Study: Antamina boosts throughput for hard ores

Introduction

• The ore body that Compania Minera Antamina has been mining in Peru since 2001 contains two principal ore types, copper molybdenum ores and much harder copper zinc ores which exist about 70 : 30 ratio.

• Historically the copper zinc ores were processed at a far slower rate and it was clear that something needed to be done.

• A collaboration between Metso Process Technology and Innovation and the Mine began in 2007 which aimed to optimise the entire comminution process.

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• The team began by auditing the drill and blast practice as well as sampling the crushing and grinding circuit.

• This helped them to develop models that would reveal what each step was achieving and what could be tweaked to improve performance

• The mine and the processing plant was then benchmarked.

• The models were calibrated and then a number of scenarios of operating strategies for both mine and process plant were run.

• An in-depth review of existing practices were carried out. 208

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• The ore was categorised in varying groups of hardness.

• Blast practices were audited and blast fragments were measured which made it possible to benchmark existing practices, and to define the main constraints related to wall stability and control ore dilution and environmental aspects.

• Site specific models for the comminution process was created and it became evident that the largest potential gains to the blast could be found.

• The basic idea was to increase the powder factor using more explosives to create a finer ROM fragmentation so that downstream equipment would treat the ore with ease.

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• In the drilling process the drill pattern ( burden and spacing) was reduced• By maintaining the same

type and amount of explosives in each drill hole, the corresponding blast powder factor rose from 0.35 - 0.54 kg/ton

• In addition switching to electronic detonators proved to be more reliable and ensured that blasts went off according to plan.• A pebble crusher was

also installed and modification to the pulp lifters were made. 210

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Conclusion

• Mine to mill optimisation work increased throughput by 30 %

• Process plant improvements contributed 10 % increase in throughput

• Reduction in hardness of the copper zinc ore contributed 15% to the increase in throughput

• As of 2011 Antamina was processing copper zinc ores at an average rate of 4400 tons per hour, up 60 % from the performance prior to 2007

• The copper – molybdenum ore also saw an increase to 4800 tons per hour.

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CASE STUDY : BATU HIJAU (INDONESIA)

PRODUCTION PLANNING FOR THE COMBINED MINE TO MILL OPERATION

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Case Study: Introduction

• The Batu Hijau copper – gold operation commenced a mine to mill program in 2001 with the standard initial objective:

• To modify blast practice to improve SAG mill throughput. • The work presented

spans over 10 years of development.

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Case Study Batu Hijau

• Using rock mass characterisation data, ore hardness and blast design data, simple regression models were developed which predicted SAG mill throughput.

• This was done for different zones in the ore body ultimately resulting in separate throughput predictions for 16 ore body domains.

• JKSimMet was used to enhance the initial regression models in order to more accurately predict the expected SAG mill throughput for the different domains.

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Case Study : Batu Hijau

• Attention then turned to developing the best blasting practice for the domains to reduce fragmentation top size in order to improve loading rates in the pit and increase grinding circuit throughput.

• Different blast designs were developed for each domain.• The modelling approach

also provided a basis for ore scheduling and production forecasting

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• The second phase of the study was based on improving prediction of mill throughput based on improved orebody characterisation.• Improving prediction of

blasting performance and refining mill models.

• The other major advance has been the use of the modelling approach for both short and long term production planning.

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• In 2007 the equations linking mill throughput to measurable variables were coded into the mine block model so that throughput predictions became a direct output from the block models.

• As previously the throughput relations were based on regression models of the tph as a function of characterisation variables.

• In effect the models established a benchmark performance which can be expected when mining and processing ore from different domains. 217

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Outcomes first phase

• Productivity gains of 10% for loading rates in the pit and 10-15% increases in SAG mill throughput for the individual ore domains were reported. Some of the important requirements for the effective implementation of the Batu Hijau M2M

• Strategy included:• The need for a dedicated

team of involved staff from geology, mining, milling IT support

• Strong and on-going support of senior management

• Best possible orebody characterisation – an on-going requirement with continuing updating of the domain models

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Outcome first phase

• Accurate models of blasting and comminution to establish expected performance for each domain and the best balance in cost and effort between blasting and milling for each domain

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Outcome second phase

• The second phase of the Batu Hijau study provides the basis for a much wider range of M2M applications than just increasing SAG mill throughput.

• There is a demonstrated ability to predict mill throughput over the long term to +/-2% accuracy

• At the core of the latest developments is a greater ability to predict mill throughput with considerable accuracy for different ore sources.

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• The applications of that capability include:• The understanding of the

expected or benchmark performance against which actual performance can be compared.

• Deviations from the expected can be identified and remedial action to regain performance can be better targeted

• The availability of a sound basis on which improvements in the grinding circuit can be identified, implemented and measured

• A tool which is an integral part of both long and short term production planning to achieve required production rates

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ORE DRESSING STUDIES – WHAT IS INVOLVED

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ODS - What is involved

Introduction

• Ore dressing studies the characterisation of the ore body with respect to metallurgical properties.

• In conjunction with the project requirements, geologists and mineral resource management, a sampling program is compiled for the specific ore body.

• These samples are characterised with respect to various flowsheet and data obtained from the characterisation work is analysed and evaluated to improve the process recovery .

• This provides information with regards to risk minimisation, for both plant design envelopes as well as operational efficiency 223

ODS - Knowledge flow

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ODS - In an ore body development

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ODS - Generic diagram for sample characterisation

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ODS - Comminution Characterisation

• Test work consists of a suit of laboratory and pilot plant scale tests

• Laboratory tests are typically rock mechanic tests as used by equipment manufacturers to provide performance guarantees for comminution equipment.

• These also include drop weight tests , a methodology used to determine the extent of breakage resistance due to impact and abrasion.

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ODS - Comminution Characterisation

• Depending on the requirement of the specific ore dressing study, i.e. feasibility study , pilot scale tests can be conducted on various comminution equipment to validate laboratory scale test results and generate plant design information.

• Samples can also be provided to equipment manufactures to conduct their own tests

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ODS - Data Analysis and Interpretation

• The data generated from the characterisation tests is analysed and interpreted by process specialists.

• This is a collaborated effort amongst in-house specialists, proprietary and commercial software, research institutes, and equipment manufactures and suppliers.

• Interpretation in this context means that key metallurgical parameters are determined and operating envelopes are established.

• Also potentially problematic ore types are identified and process recommendations are made. 229

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ODS - Data Analysis and Interpretation

• The output results in key plant design information.• E.g. comminution

characterisation predicts the product size distribution and mass balance via simulation for scrubbing and each of the crushing stages.

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ODS - Integration

• The role of the metallurgist is key in generating the flowsheet design knowledge package through the interaction with a variety of process specialists and process engineers.

• Important major ore related problem areas within a specific ore type are also highlighted.

• This means that such problem areas and solutions are integrated within the overall process design.

• Depending on the phase of the project the integration process also includes a level of simulation of the ore dressing study, and derived flowsheet options that resulted from the characterisation of the various ore types. 231

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ODS - Integration

• Simulation enables critical investigation of all system attributes, and the ability of the circuit design to deliver finished product with out recycling.

• Raw ore dressing information and knowledge is traded off against practical operational constraints, which leads to a fit-for-purpose design

• That has the best chance of maximizing recovery of minerals from in-situ resources.

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ODS - Plant design

• To reduce the risk of selecting incorrect equipment from a vast array of possibilities a formalised set of tools to guide equipment selection and plant design have been developed

• These tools consist of commercially available as well as proprietary tools

• Process engineers are provided with basic flow diagrams and related metallurgical parameters.

• The process engineer will then expand on the original ore dressing flowsheet provided and develop a number of flowsheet based on the project requirements. 233

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ODS - Plant design

• Completed ore dressing study assists the process engineer to rapidly evaluate scenarios using existing models and create an understanding of how the metallurgical envelope of characteristics develop through the ore body.

• An evaluation of proposed solutions against a background of knowledge derived from the study is then conducted.

• The knowledge derived from the study supports the engineer in the design phase and assists in reducing project risk and increases confidence in the approved flowsheet. 234

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PROFIT BASED GRINDING CONTROLSCase Study : SierritaProgram controller : Duval Corporation

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Case Study : Sierrita

Introduction

• The comminution circuit represents the largest user of energy in the Mineral Processing Industry• As the grades of ore

reduces the economics of energy usage becomes more significant.

• Pertinent control theory for the control of comminution circuits has been known for a long time but it is of recent years that practical techniques and robust computer control architectures for these systems have become available 236


Description of the Plant

• Wet grinding circuit treats 90 000 stpd• Sixteen x Allis-Chalmers

overflow ball mills are operated in parallel in a conventional closed circuit wet grinding system.

• The very low grade ore, variable crusher product, and changes in ore hardness produces disturbances that upset the performance of the grinding circuit.

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Instrumentation

• Variable feeder• Feeder Weight

measurement system• Feed water flow meter• Sump water flow rate• Sump level indicator

• Mill power draft• Pump amperage• Control valves to feed

and sump water• All the other variables

are calculated using inferential techniques

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Process Analysis study

• Fig. 2 shows a schematic diagram of a ball mill / cyclone control system

• This diagram shows the instrumentation and the calculated variables use in the control strategy.

• From the process analysis study several objectives were established. 239

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Objectives

• Reduction of mill feed size• Reduction of mill power

consumption• Extending mill transport

conditions

• Investment in variable speed drives • Identification of proper

linking of manipulated variables with control variables• Identification of inferred

measurements and signal conditioning of the raw measurements 240

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Disturbances

• Mill feed particle size distribution due to bin segregation and crusher circuit operation• Ore hardness and ore

mineralogical structure and composition due to natural mining characteristics

• Pumping / classification limitations and equipment wear.

• Process analysis study showed that the calculations of the inferred calculated variables can provide adequate information for the development of control strategy

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Case Study : SierritaDesign of the plant control

strategy

• Fig. 3 shows that for a given ore there is a unique milling rate to provide the grind size that will yield maximum profit under certain economic conditions

• A higher milling rate can be achieved with a coarser grind which is off set by losses in recovery due to poor liberation

• A finer grind producers better recoveries but loss in throughput rate.• Swings outside the given

band produces losses

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Design of plant control strategy

• Fig. 4 shows the control objectives and limiting conditions that the control system must overcome to produce a profit.

• The main controller is the mill load constraint controller for safe operation followed by the grind cut controller for profitable operation

• The mill load constraint controller involves the changing mill transport constraint and sets the tonnage for feasible operation

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Design of plant control strategy

• The grind cut inferential controller maintains the optimal liberation, if process operational limits permit.• The curves depicted in

fig. 3 and 4 are not unique and are changing constantly.

• Thus the information must be handled on a timely basis in a computer system.

• The computer system will in turn provide for adapting values of the moving constraint and set points. This known as online adaptive decision making or control.

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Grinding controls

• The primary objective of the grinding controls system is to provide a flexible , adaptive, easy to use system to:

• Maintain an optimal throughput depending on the ore conditions. This will provide the downstream process with a constant size distribution for improved recovery.

• Or to maintain a stable operation while assisting the operator in maximizing the throughput, avoiding frequent upsets or spills and maintaining an adequate grind.

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Grinding controlsSimplified function block control

strategy

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The four principle controllers

• The ball mill load control system• The grind index control

strategy• The ball mill transport

index control strategy• Sump level controller

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The ball mill load controller

• This is the main controller• Any additional capacity

of the ball mill depending on the grind setting is sensed by the ball mill load controller and the feed rate is increased.

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Control Design

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Case Study : SierritaMAIN CONTROLLER WINDOW

DISPLAY

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Case Study : SierritaComputer Architecture for plant

management

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Overall plant control strategy

• The current objective of increasing the recovery / profit by running an optimal throughput can be enhanced by proper co-ordination of the plant activities.

• Fig. 8 shows the computer architecture used to integrate the distributed control system with process management activities

• Four process control units are networked to two operator interface units.

• The plant host computer is also used for engineering analysis of operating and lab oratory information with statistical modelling, process analysis and simulation and reporting software packages. 252

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Conclusions

• A profitability concept was transformed into a feasible mode of operation.• The tonnage setting can

be safely pushed up to 400 stph from 250 stph while maintaining metallurgical performance. 253

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INCREASING THE ENERGY EFFICIENCY OF PROCESSING

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Mineral Processing - Crushing - Plant design, construction, operation and optimisation

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Transcript of Mineral Processing - Crushing - Plant design, construction, operation and optimisation